ellipsis()
  overflow: hidden
  white-space: nowrap
  text-overflow: ellipsis
bgimage($url)
  background-image: url($url + '@2x.png')  //传入参数后自动拼接
  @media screen and (-webkit-min-device-pixel-ratio: 3), (min-device-pixel-ratio: 3)
    background-image: url($url + '@3x.png')
